The arrangements discussed here concern motors of the type disclosed in U.S. Pat. No. 4,673,849, issued on Jun. 16, 1987, to Sears et al. for a Permanent Magnet Motor Closed Loop Restarting System; and U.S. Pat. No. 4,818,922, issued on Apr. 4, 1989, to Sears et al. for Optical Commutation for Permanent Magnet Motors; both incorporated by reference herein.
Accurate speed control is a desirable element in the operation of permanent magnet motors, as for example in gyroscopic applications. By controlling the signal applied to the drive winding (the .phi.A winding) of a motor, the speed of the rotor can be controlled. However, the drive signal is not a constant and must be varied to account for changes in drag due to windage, friction, and other factors to avoid over- and underspeed conditions.
A suitable control signal can be applied using dual-polarity pulse width modulation. The phase of the back emf generated in the motor's sensor winding (the .phi.B winding) is compared against the phase of the command signal. In turn, a variable duty-cycle pulse-width modulation signal is generated, where the duty cycle is proportional to the phase difference.